KENNEDY SPACE CENTER, FL--Engineers have eliminated one of three damage sites on the shuttle Endeavour's heat shield tiles as an area of concern and a second may be dismissed and deemed safe for re-entry as is by Friday, a senior manager said Thursday. But one damage site on the shuttle's belly may require an additional, "focused," inspection Saturday to make sure repairs are not needed.

LeRoy Cain, chairman of NASA's Mission Management Team, made it clear during a status briefing Thursday that he was not overly concerned by the damage to Endeavour's heat shield tiles, saying the primary area of interest was very similar to a tile gouge seen during a 2007 mission by Endeavour. In that case, a focused inspection was carried out and engineers determined, correctly as it turned out, that the shuttle could safely re-enter without the need for any repairs.

Two damage sites labeled with yellow boxes remain areas of concern for engineers assessing whether or not to carry out a so-called focused inspection of the shuttle Endeavour's heat shield Saturday. (Credit: NASA TV)

Addressing the "worry factor" for Endeavour's current mission, Cain said "when I was here yesterday, I talked about the fact that I am not concerned about the damage that we're seeing here, it's certainly not alarming. And the team is not concerned about it. My confidence is largely derived from the fact that we have a very good process for dispositioning these things, we know exactly how to go assess them, we've gotten better and better at it and we know that our models are doing nothing but improving as time goes on and we get more experience.

"In the event that we do come up to a point in our assessment where we feel like we need to get some more data, we know how to go do that, i.e., focused inspection, which we're talking about doing now. ... And then when it comes right down to it, if focused inspection and the analysis of the data from focused inspection determine we didn't have the kind of margins here that we wanted to have, then we do have some options to go do some repair."

In the wake of the 2003 Columbia disaster, NASA developed tile repair tools that range from STA-54, a thick material applied with a grease gun-like applicator that could be used by spacewalking astronauts to fill in a tile gouge, to a heat-resistant overlay that could be bolted in place over a larger damage site.

"But that's really about as far as I want to take this discussion because I feel pretty confident if, in fact, we're not able to clear it by the morning, when we get the focused inspection data we'll be able to clear this problem and not have to do anything. But having said that, we're going to go do whatever is necessary and we're going to follow the data all the way along."

During Endeavour's approach to the International Space Station Wednesday, commander Mark Kelly guided the ship through a computer-assisted back-flip maneuver to expose the shuttle's belly to the lab complex. Station astronauts Catherine Coleman, Paolo Nespoli and Expedition 27 commander Dmitry Kondratyev, photographed the shuttle's heat shield with powerful telephoto lenses to help engineers look for any signs of damage.

The photographs showed seven damage sites under the right wing running in a rough line from the right-side main landing gear door to an aft elevon, or wing flap. As of Wednesday, four of the seven sites had been ruled safe for entry as is and by Thursday, one of the remaining sites was eliminated, leaving just two areas of concern.

One of those, a gouge running across the hingeline of Eneavour's right-side inboard elevon, may be eliminated by Friday, Cain said. The other site, located between the right main landing gear door and an external tank liquid oxygen feel line port, may be more difficult to assess without a focused inspection.

The gouge measures 3.22 inches by 2.49 inches and is believed to be about seven tenths of an inch deep. The 2007 impact cited earlier by Cain resulted in a gouge measuring 3.48 inches by 2.31 inches with a depth of 1.12 inches. In that case, engineers eventually cleared Endeavour for re-entry as is and the shuttle completed a safe landing.

"In summary, we have some more work to do on a couple of these sites," Cain said. "Petty high confidence we're going to clear one of them, some possibility we're going to clear the other one. We have procedures built and timelines built and ready to go to the crew tomorrow morning whatever the scenario might be, for one or both of the sites. Or neither of them."

In a focused inspection, a camera on the end of the space station's robot arm would be used to carefully inspect a damage site of interest. If such an inspection is ordered for Endeavour, the astronauts would use a three-hour block of off-duty time early Saturday, starting around 2:26 a.m. EDT (GMT-4).